This invention relates to a method for the production of a low density polyethylene having a density of not higher than 0.945 g/cm.sup.3 (according to ASTM D-1505) by copolymerization in slurry state of ethylene and an .alpha.-olefin in the presence of a high activity catalyst.
When a slurry polymerization by the so-called, medium pressure-low pressure polymerization process is applied for the production of a low density polyethylene, a considerable amount (more than 5 weight %) of the resulting polymer is extracted in the polymerization solvent so that the bulk density of polymer particles is extremely reduced, and the polymer slurry becomes viscous and accordingly, advancing of polymerization reaction encounters difficulties. Particularly, it was impossible to produce low density polyethylenes of not higher than 0.927 g/cm.sup.3 by the slurry polymerization. Accordingly, low density grades having the density range of from 0.915 to 0.927 g/cm.sup.3, which corresponds to that of the low density polyethylene from the so-called, high pressure polymerization process have, hitherto, been not handled in the catalogues of polyethylene products from the so-called, low pressure polymerization process. By way of Example, according to Hydrocarbon Processing, Nov. (1972) 97 and ibid Nov. (1973) 167, the density of products is limited to the range of 0.940-0.965 g/cm.sup.3 or 0.947-0.969 g/cm.sup.3.
On the other hand, use of supported catalysts with a high activity has been proposed to obtain the polymer slurry easy to handle. For example, U.S. Pat. No. 3,888,835 discloses a process for the polymerization of ethylene, in which a high activity catalyst having a titanium compound supported on a magnesium compound is used and such a low-boiling hydrocarbon as butane is used as a diluent for polymerization.
However, when this process is applied to the production of the low density polyethylene having a density of 0.935 g/cm.sup.3 or less through copolymerization of ethylene with an .alpha.-olefin, the resulting polymer is reduced in bulk density and thus it is difficult to obtain the polymer slurry easy to handle. Particularly, the lowering of the polymer bulk density results in lowering of an agitation efficiency of the reactor for polymerization unless the resulting polymer concentration per unit quantity of the polymerization solvent used is controlled at a low level, and thus the dissolution velocity of ethylene into the solvent is lowered so that advancing of polymerization reaction becomes substantially impossible.
Further, the lowering of bulk density or the formation of bulky polymer particles brings about troubles in the after-treatment of the polymer slurry, for example, filtrating of the slurry, separating of the solvent and drying of the polymer particles encounter a great difficulty.
Furthermore, in the producing of the low density polyethylene by the slurry polymerization, it was exceedingly difficult to avoid such troubles as deposition of polymers on the wall of polymerization equipments and sedimentation of coarse polymer particles.